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NAME

       uri, url, urn - uniform resource identifier (URI), including a URL or URN

SYNOPSIS

       URI = [ absoluteURI | relativeURI ] [ "#" fragment ]

       absoluteURI = scheme ":" ( hierarchical_part | opaque_part )

       relativeURI = ( net_path | absolute_path | relative_path ) [ "?" query ]

       scheme = "http" | "ftp" | "gopher" | "mailto" | "news" | "telnet" |
                  "file" | "man" | "info" | "whatis" | "ldap" | "wais" | ...

       hierarchical_part = ( net_path | absolute_path ) [ "?" query ]

       net_path = "//" authority [ absolute_path ]

       absolute_path = "/"  path_segments

       relative_path = relative_segment [ absolute_path ]

DESCRIPTION

       A  Uniform  Resource  Identifier  (URI)  is  a  short  string of characters identifying an
       abstract or physical resource (for example, a web page).  A Uniform Resource Locator (URL)
       is  a  URI  that  identifies  a  resource  through its primary access mechanism (e.g., its
       network "location"), rather than by name or some other  attribute  of  that  resource.   A
       Uniform  Resource Name (URN) is a URI that must remain globally unique and persistent even
       when the resource ceases to exist or becomes unavailable.

       URIs are the standard way to name hypertext  link  destinations  for  tools  such  as  web
       browsers.   The  string "http://www.kernelnotes.org" is a URL (and thus it is also a URI).
       Many people use the term URL loosely as a synonym for URI (though technically URLs  are  a
       subset of URIs).

       URIs can be absolute or relative.  An absolute identifier refers to a resource independent
       of context, while a relative identifier refers to a resource by describing the  difference
       from  the  current  context.  Within a relative path reference, the complete path segments
       "." and ".." have special meanings: "the current hierarchy level"  and  "the  level  above
       this  hierarchy  level",  respectively,  just  like  they do in UNIX-like systems.  A path
       segment which contains a colon character can't be used as the first segment of a  relative
       URI path (e.g., "this:that"), because it would be mistaken for a scheme name; precede such
       segments with ./ (e.g., "./this:that").  Note that descendants of MS-DOS (e.g.,  Microsoft
       Windows)  replace  devicename  colons with the vertical bar ("|") in URIs, so "C:" becomes
       "C|".

       A fragment identifier, if included, refers to a particular named portion (fragment)  of  a
       resource;  text  after  a '#' identifies the fragment.  A URI beginning with '#' refers to
       that fragment in the current resource.

   Usage
       There are many different URI schemes, each with specific additional  rules  and  meanings,
       but  they  are  intentionally  made  to  be as similar as possible.  For example, many URL
       schemes permit the authority to be the following format, called here an ip_server  (square
       brackets show what's optional):

       ip_server = [user [ : password ] @ ] host [ : port]

       This  format  allows  you  to optionally insert a username, a user plus password, and/or a
       port number.  The host is the name of the host computer, either its name as determined  by
       DNS    or    an   IP   address   (numbers   separated   by   periods).    Thus   the   URI
       <http://fred:fredpassword@xyz.com:8080/> logs into a web server on host  xyz.com  as  fred
       (using  fredpassword)  using  port  8080.  Avoid including a password in a URI if possible
       because of the many security risks of having a password written down.  If the URL supplies
       a  username  but  no  password,  and  the  remote  server requests a password, the program
       interpreting the URL should request one from the user.

       Here are some of the most common schemes in use on UNIX-like systems that  are  understood
       by  many tools.  Note that many tools using URIs also have internal schemes or specialized
       schemes; see those tools' documentation for information on those schemes.

       http - Web (HTTP) server

       http://ip_server/path
       http://ip_server/path?query

       This is a URL accessing a web (HTTP) server.  The default port is 80.  If the path  refers
       to  a  directory,  the  web  server will choose what to return; usually if there is a file
       named "index.html" or "index.htm" its content is returned, otherwise, a list of the  files
       in  the  current directory (with appropriate links) is generated and returned.  An example
       is <http://lwn.net>.

       A query can be given in the archaic "isindex" format, consisting of a word or  phrase  and
       not  including  an  equal sign (=).  A query can also be in the longer "GET" format, which
       has one or more query entries of the form key=value separated by the  ampersand  character
       (&).   Note  that key can be repeated more than once, though it's up to the web server and
       its application programs to determine if  there's  any  meaning  to  that.   There  is  an
       unfortunate  interaction  with HTML/XML/SGML and the GET query format; when such URIs with
       more than one key are embedded in SGML/XML documents (including HTML), the  ampersand  (&)
       has to be rewritten as &amp;.  Note that not all queries use this format; larger forms may
       be too long to store as a URI, so they use a different interaction mechanism (called POST)
       which  does  not  include  the  data  in  the  URI.   See  the  Common  Gateway  Interface
       specification at <http://www.w3.org/CGI> for more information.

       ftp - File Transfer Protocol (FTP)

       ftp://ip_server/path

       This is a URL accessing a file through the file transfer protocol (FTP).  The default port
       (for  control)  is  21.  If no username is included, the username "anonymous" is supplied,
       and in that case many clients provide as  the  password  the  requestor's  Internet  email
       address.  An example is <ftp://ftp.is.co.za/rfc/rfc1808.txt>.

       gopher - Gopher server

       gopher://ip_server/gophertype selector
       gopher://ip_server/gophertype selector%09search
       gopher://ip_server/gophertype selector%09search%09gopher+_string

       The  default  gopher  port  is  70.   gophertype is a single-character field to denote the
       Gopher type of the resource to which the URL refers.  The entire path may also  be  empty,
       in which case the delimiting "/" is also optional and the gophertype defaults to "1".

       selector  is  the Gopher selector string.  In the Gopher protocol, Gopher selector strings
       are a sequence of octets which may contain any octets except 09 hexadecimal  (US-ASCII  HT
       or tab), 0A hexadecimal (US-ASCII character LF), and 0D (US-ASCII character CR).

       mailto - Email address

       mailto:email-address

       This  is  an  email  address, usually of the form name@hostname.  See mailaddr(7) for more
       information on the correct format of an email address.  Note that any % character must  be
       rewritten as %25.  An example is <mailto:dwheeler@dwheeler.com>.

       news - Newsgroup or News message

       news:newsgroup-name
       news:message-id

       A     newsgroup-name    is    a    period-delimited    hierarchical    name,    such    as
       "comp.infosystems.www.misc".  If <newsgroup-name> is "*" (as in <news:*>), it is  used  to
       refer to "all available news groups".  An example is <news:comp.lang.ada>.

       A     message-id     corresponds     to     the     Message-ID     of    IETF    RFC 1036,
       ⟨http://www.ietf.org/rfc/rfc1036.txt⟩ without the enclosing "<" and ">"; it takes the form
       unique@full_domain_name.  A message identifier may be distinguished from a news group name
       by the presence of the "@" character.

       telnet - Telnet login

       telnet://ip_server/

       The Telnet URL scheme is used to designate interactive text services that may be  accessed
       by  the Telnet protocol.  The final "/" character may be omitted.  The default port is 23.
       An example is <telnet://melvyl.ucop.edu/>.

       file - Normal file

       file://ip_server/path_segments
       file:path_segments

       This represents a file or directory accessible locally.  As a special case,  host  can  be
       the string "localhost" or the empty string; this is interpreted as "the machine from which
       the URL is being interpreted".  If the path is to a directory, the viewer  should  display
       the  directory's contents with links to each containee; not all viewers currently do this.
       KDE supports generated files through the URL <file:/cgi-bin>.  If  the  given  file  isn't
       found,  browser  writers may want to try to expand the filename via filename globbing (see
       glob(7) and glob(3)).

       The second format (e.g., <file:/etc/passwd>) is a correct format for referring to a  local
       file.   However,  older  standards  did  not  permit  this format, and some programs don't
       recognize this as a URI.  A more portable syntax is to use an empty string as  the  server
       name,  for  example,  <file:///etc/passwd>;  this  form  does the same thing and is easily
       recognized by pattern matchers and older programs as a URI.  Note that if you really  mean
       to  say "start from the current location," don't specify the scheme at all; use a relative
       address like <../test.txt>, which has the side-effect  of  being  scheme-independent.   An
       example of this scheme is <file:///etc/passwd>.

       man - Man page documentation

       man:command-name
       man:command-name(section)

       This refers to local online manual (man) reference pages.  The command name can optionally
       be followed by a parenthesis and section number; see man(7) for more  information  on  the
       meaning  of  the section numbers.  This URI scheme is unique to UNIX-like systems (such as
       Linux) and is not currently registered by the IETF.  An example is <man:ls(1)>.

       info - Info page documentation

       info:virtual-filename
       info:virtual-filename#nodename
       info:(virtual-filename)
       info:(virtual-filename)nodename

       This scheme refers to online info  reference  pages  (generated  from  texinfo  files),  a
       documentation format used by programs such as the GNU tools.  This URI scheme is unique to
       UNIX-like systems (such as Linux) and is not currently registered by the IETF.  As of this
       writing,  GNOME  and  KDE differ in their URI syntax and do not accept the other's syntax.
       The first two formats are the GNOME  format;  in  nodenames  all  spaces  are  written  as
       underscores.   The  second  two  formats  are  the KDE format; spaces in nodenames must be
       written as spaces, even though this is forbidden by the URI standards.  It's hoped that in
       the  future  most  tools  will  understand  all  of  these  formats and will always accept
       underscores for spaces in nodenames.  In both GNOME and  KDE,  if  the  form  without  the
       nodename  is  used  the nodename is assumed to be "Top".  Examples of the GNOME format are
       <info:gcc> and <info:gcc#G++_and_GCC>.  Examples of the KDE format  are  <info:(gcc)>  and
       <info:(gcc)G++ and GCC>.

       whatis - Documentation search

       whatis:string

       This scheme searches the database of short (one-line) descriptions of commands and returns
       a list of descriptions containing that string.  Only complete word matches  are  returned.
       See  whatis(1).  This URI scheme is unique to UNIX-like systems (such as Linux) and is not
       currently registered by the IETF.

       ghelp - GNOME help documentation

       ghelp:name-of-application

       This loads GNOME help for  the  given  application.   Note  that  not  much  documentation
       currently exists in this format.

       ldap - Lightweight Directory Access Protocol

       ldap://hostport
       ldap://hostport/
       ldap://hostport/dn
       ldap://hostport/dn?attributes
       ldap://hostport/dn?attributes?scope
       ldap://hostport/dn?attributes?scope?filter
       ldap://hostport/dn?attributes?scope?filter?extensions

       This  scheme  supports  queries  to  the  Lightweight  Directory Access Protocol (LDAP), a
       protocol for querying a set of servers for hierarchically organized information  (such  as
       people  and computing resources).  More information on the LDAP URL scheme is available in
       RFC 2255.  ⟨http://www.ietf.org/rfc/rfc2255.txt⟩ The components of this URL are:

       hostport    the LDAP server to query, written as a hostname optionally followed by a colon
                   and  the  port  number.  The default LDAP port is TCP port 389.  If empty, the
                   client determines which the LDAP server to use.

       dn          the LDAP Distinguished Name, which identifies the  base  object  of  the  LDAP
                   search (see RFC 2253 ⟨http://www.ietf.org/rfc/rfc2253.txt⟩ section 3).

       attributes  a  comma-separated  list  of  attributes  to be returned; see RFC 2251 section
                   4.1.5.  If omitted, all attributes should be returned.

       scope       specifies the scope of the search, which can be one  of  "base"  (for  a  base
                   object  search),  "one"  (for  a  one-level  search),  or "sub" (for a subtree
                   search).  If scope is omitted, "base" is assumed.

       filter      specifies the search filter (subset of entries to return).   If  omitted,  all
                   entries         should         be        returned.         See        RFC 2254
                   ⟨http://www.ietf.org/rfc/rfc2254.txt⟩ section 4.

       extensions  a comma-separated list of type=value pairs, where the =value  portion  may  be
                   omitted  for  options  not  requiring it.  An extension prefixed with a '!' is
                   critical (must  be  supported  to  be  valid),  otherwise  it  is  noncritical
                   (optional).

       LDAP   queries   are   easiest   to   explain  by  example.   Here's  a  query  that  asks
       ldap.itd.umich.edu for information about the University of Michigan in the U.S.:

       ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US

       To just get its postal address attribute, request:

       ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US?postalAddress

       To ask a host.com at port 6666 for information about the  person  with  common  name  (cn)
       "Babs Jensen" at University of Michigan, request:

       ldap://host.com:6666/o=University%20of%20Michigan,c=US??sub?(cn=Babs%20Jensen)

       wais - Wide Area Information Servers

       wais://hostport/database
       wais://hostport/database?search
       wais://hostport/database/wtype/wpath

       This   scheme  designates  a  WAIS  database,  search,  or  document  (see  IETF  RFC 1625
       ⟨http://www.ietf.org/rfc/rfc1625.txt⟩ for more information  on  WAIS).   Hostport  is  the
       hostname, optionally followed by a colon and port number (the default port number is 210).

       The  first  form  designates  a WAIS database for searching.  The second form designates a
       particular search of the WAIS database database.  The third form designates  a  particular
       document  within  a  WAIS  database to be retrieved.  wtype is the WAIS designation of the
       type of the object and wpath is the WAIS document-id.

       other schemes

       There are many other URI schemes.  Most tools that accept URIs support a set  of  internal
       URIs  (e.g.,  Mozilla  has  the about: scheme for internal information, and the GNOME help
       browser has the toc: scheme for various starting locations).  There are many schemes  that
       have  been  defined  but are not as widely used at the current time (e.g., prospero).  The
       nntp: scheme is deprecated in favor of the news: scheme.  URNs are to be supported by  the
       urn:  scheme,  with  a  hierarchical  name  space  (e.g., urn:ietf:... would identify IETF
       documents); at this time URNs are not widely  implemented.   Not  all  tools  support  all
       schemes.

   Character Encoding
       URIs use a limited number of characters so that they can be typed in and used in a variety
       of situations.

       The following characters are reserved, that is, they may appear in a URI but their use  is
       limited  to  their  reserved  purpose (conflicting data must be escaped before forming the
       URI):

                 ; / ? : @ & = + $ ,

       Unreserved characters may be included in a URI.  Unreserved characters include  upper  and
       lower  case  English letters, decimal digits, and the following limited set of punctuation
       marks and symbols:

               - _ . ! ~ * ' ( )

       All other characters must be escaped.  An escaped octet is encoded as a character triplet,
       consisting   of  the  percent  character  "%"  followed  by  the  two  hexadecimal  digits
       representing the octet code (you can use upper or lower case letters for  the  hexadecimal
       digits).   For  example, a blank space must be escaped as "%20", a tab character as "%09",
       and the "&" as "%26".  Because the percent "%" character always has the  reserved  purpose
       of  being  the  escape  indicator,  it must be escaped as "%25".  It is common practice to
       escape space characters as the  plus  symbol  (+)  in  query  text;  this  practice  isn't
       uniformly  defined  in  the  relevant  RFCs  (which  recommend  %20  instead) but any tool
       accepting URIs with query text should be prepared for them.  A URI is always shown in  its
       "escaped" form.

       Unreserved  characters  can be escaped without changing the semantics of the URI, but this
       should not be done unless the URI is being used in a  context  that  does  not  allow  the
       unescaped  character to appear.  For example, "%7e" is sometimes used instead of "~" in an
       HTTP URL path, but the two are equivalent for an HTTP URL.

       For URIs which must handle characters outside the US ASCII character set,  the  HTML  4.01
       specification  (section  B.2)  and  IETF  RFC 2718 (section 2.2.5) recommend the following
       approach:

       1.  translate the character sequences into UTF-8 (IETF RFC 2279)—see utf-8(7)—and then

       2.  use the URI escaping mechanism, that is, use the %HH encoding for unsafe octets.

   Writing a URI
       When    written,    URIs    should    be    placed    inside    double    quotes    (e.g.,
       "http://www.kernelnotes.org"),  enclosed  in  angle  brackets (e.g., <http://lwn.net>), or
       placed on a line by themselves.  A warning for those who  use  double-quotes:  never  move
       extraneous  punctuation  (such  as  the  period  ending a sentence or the comma in a list)
       inside a URI, since this will change the value of the URI.  Instead,  use  angle  brackets
       instead,  or  switch  to a quoting system that never includes extraneous characters inside
       quotation marks.  This latter system, called the 'new'  or  'logical'  quoting  system  by
       "Hart's  Rules" and the "Oxford Dictionary for Writers and Editors", is preferred practice
       in Great Britain and hackers worldwide (see the Jargon File's section  on  Hacker  Writing
       Style, http://www.fwi.uva.nl/~mes/jargon/h/HackerWritingStyle.html, for more information).
       Older documents suggested inserting the prefix "URL:" just before the URI, but  this  form
       has never caught on.

       The  URI syntax was designed to be unambiguous.  However, as URIs have become commonplace,
       traditional media (television, radio, newspapers, billboards, etc.) have increasingly used
       abbreviated  URI  references  consisting  of  only  the authority and path portions of the
       identified  resource  (e.g.,  <www.w3.org/Addressing>).   Such  references  are  primarily
       intended  for  human interpretation rather than machine, with the assumption that context-
       based heuristics are sufficient to complete the URI (e.g., hostnames beginning with  "www"
       are  likely to have a URI prefix of "http://" and hostnames beginning with "ftp" likely to
       have a prefix of "ftp://").   Many  client  implementations  heuristically  resolve  these
       references.   Such  heuristics  may  change  over  time, particularly when new schemes are
       introduced.  Since an abbreviated URI  has  the  same  syntax  as  a  relative  URL  path,
       abbreviated  URI references cannot be used where relative URIs are permitted, and can only
       be used when there is no defined base (such as in dialog boxes).   Don't  use  abbreviated
       URIs as hypertext links inside a document; use the standard format as described here.

CONFORMING TO

       http://www.ietf.org/rfc/rfc2396.txt (IETF RFC 2396), http://www.w3.org/TR/REC-html40 (HTML
       4.0).

NOTES

       Any tool accepting URIs (e.g., a web browser) on a Linux system should be able  to  handle
       (directly  or  indirectly) all of the schemes described here, including the man: and info:
       schemes.  Handling them by invoking some other program is fine and in fact encouraged.

       Technically the fragment isn't part of the URI.

       For information on how to embed URIs (including URLs) in a data format, see  documentation
       on  that  format.   HTML  uses the format <A HREF="uri"> text </A>.  Texinfo files use the
       format @uref{uri}.  Man and mdoc have the recently added UR macro, or just include the URI
       in the text (viewers should be able to detect :// as part of a URI).

       The  GNOME  and  KDE  desktop  environments  currently  vary  in  the URIs they accept, in
       particular in their respective help browsers.  To list man  pages,  GNOME  uses  <toc:man>
       while KDE uses <man:(index)>, and to list info pages, GNOME uses <toc:info> while KDE uses
       <info:(dir)> (the author of this man page prefers the KDE approach  here,  though  a  more
       regular  format  would be even better).  In general, KDE uses <file:/cgi-bin/> as a prefix
       to a set of generated  files.   KDE  prefers  documentation  in  HTML,  accessed  via  the
       <file:/cgi-bin/helpindex>.    GNOME   prefers   the   ghelp   scheme  to  store  and  find
       documentation.  Neither browser handles file: references to directories  at  the  time  of
       this  writing,  making  it difficult to refer to an entire directory with a browsable URI.
       As noted above, these environments differ in how they handle the  info:  scheme,  probably
       the  most  important variation.  It is expected that GNOME and KDE will converge to common
       URI formats, and a future version of this man page will  describe  the  converged  result.
       Efforts to aid this convergence are encouraged.

   Security
       A  URI  does  not  in itself pose a security threat.  There is no general guarantee that a
       URL, which at one time located a given resource, will continue to do so.  Nor is there any
       guarantee  that  a  URL  will not locate a different resource at some later point in time;
       such a guarantee can only be obtained from the person(s) controlling  that  namespace  and
       the resource in question.

       It  is  sometimes  possible to construct a URL such that an attempt to perform a seemingly
       harmless operation, such as the retrieval of an entity associated with the resource,  will
       in  fact cause a possibly damaging remote operation to occur.  The unsafe URL is typically
       constructed by specifying a port number other than that reserved for the network  protocol
       in  question.   The client unwittingly contacts a site that is in fact running a different
       protocol.  The content of the URL contains instructions that, when  interpreted  according
       to  this  other protocol, cause an unexpected operation.  An example has been the use of a
       gopher URL to cause an unintended or impersonating message to be sent via a SMTP server.

       Caution should be used when using any URL that specifies a  port  number  other  than  the
       default for the protocol, especially when it is a number within the reserved space.

       Care  should  be  taken  when  a URI contains escaped delimiters for a given protocol (for
       example, CR and LF characters for telnet protocols) that these are  not  unescaped  before
       transmission.   This  might  violate  the  protocol,  but  avoids  the  potential for such
       characters to be used to simulate an extra operation or parameter in that protocol,  which
       might lead to an unexpected and possibly harmful remote operation to be performed.

       It is clearly unwise to use a URI that contains a password which is intended to be secret.
       In particular, the use of a password within the "userinfo" component of a URI is  strongly
       recommended  against except in those rare cases where the "password" parameter is intended
       to be public.

BUGS

       Documentation may be placed in a variety of locations, so there currently isn't a good URI
       scheme  for  general  online  documentation  in arbitrary formats.  References of the form
       <file:///usr/doc/ZZZ> don't work because different distributions  and  local  installation
       requirements  may  place  the  files  in  different directories (it may be in /usr/doc, or
       /usr/local/doc, or /usr/share, or  somewhere  else).   Also,  the  directory  ZZZ  usually
       changes  when  a version changes (though filename globbing could partially overcome this).
       Finally, using the file:  scheme  doesn't  easily  support  people  who  dynamically  load
       documentation  from  the Internet (instead of loading the files onto a local file system).
       A future URI scheme may be added (e.g., "userdoc:") to permit programs to  include  cross-
       references  to  more  detailed  documentation without having to know the exact location of
       that documentation.  Alternatively, a future version of the file-system specification  may
       specify  file  locations  sufficiently  so  that  the  file: scheme will be able to locate
       documentation.

       Many programs and file formats don't include a way to incorporate or implement links using
       URIs.

       Many  programs can't handle all of these different URI formats; there should be a standard
       mechanism to load an arbitrary URI  that  automatically  detects  the  users'  environment
       (e.g.,  text  or  graphics,  desktop  environment,  local  user preferences, and currently
       executing tools) and invokes the right tool for any URI.

SEE ALSO

       lynx(1),       man2html(1),       mailaddr(7),       utf-8(7),        IETF        RFC 2255
       ⟨http://www.ietf.org/rfc/rfc2255.txt

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.